In electrophotography, an electrostatic latent image formed on a photosensitive member is developed by a developer. One-component developers composed of a toner, and two-component developers composed of a toner and a carrier, such as glass beads and magnetic particles with or without resin coating, are known. Two-component developing is advantageous in comparison with one-component developing, because it uses a carrier which has large surface area, causing satisfactory triboelectric-charge for the toner, thereby making the charge of the toner stable and capable of holding high quality images for a long period of developing time.
Two-component developers are also preferred in certain high-speed apparatuses.
Two-component developing is also being widely used in digital electrophotographic systems where latent electrostatic images are formed onto a photosensitive member by laser beam-irradiation and the like, followed by developing the latent images.
Recently, size reduction and condensed distribution of dot units for latent image (pixel units) have been designed to satisfy the requirements for improving the resolution degree, reproducibility of highlight image and faithful color-imaging.
In particular, an important concern in the field is the achievement of a developing system which enables a faithful development of those latent image (dots comprising each image). Therefore, many proposals were made, both from the point of processing means and from the developer (toner and carrier). As for the processing means, a restriction of development gap and thinning of the layers comprising photosensitive member are effective; however, there are still problems in that cost is increased as a result of such improvements, and sufficient reliability is not yet achieved, and the like.
On the other hand, with regard to the developer side, dot reproducibility is considerably improved by using small sized toner. However, problems occur with developers using small sized toners, such as staining (smearing) in the background area, low image density, and others. And, in case of a toner having a small size used for full-color images, resins having a low softening-temperature are generally used which, in comparison with black toner, increase the spent amount on the surface of carrier, degrade the quality of developer over time, and show a tendency towards toner-scattering and background stain.
Various proposals for using small sized carrier have also been proposed. For example, Japanese Laid-open patent Publication No.58-144839 discloses a magnetic carrier for an electrophotographic developer using carrier particles which comprising ferrite particles of spinel structure, wherein the carrier particles having a average particle diameter of less than from 30 μm. The carrier, which is not covered with a resin layer, is used with a low developing electric field. Because the toner is not covered with a resin layer, the lifetime of it is short, and its developing ability is not sufficient.
Japanese granted patent No. 3029180 discloses a carrier for an electrophotographic developer using carrier particles, wherein the carrier particles have a size ranging from 15 μm to 45 μm in 50%-average diameter (D50), the content of smaller carrier particles less than 22 μm in size ranging from 1 to 20%, the content of small carrier particles less than 16 μm in size is not higher than 3%, the content of large carrier particles more than or equal to 62 μm in size ranges from 2% to 15%, the content of larger carrier particles more than 88 μm in size not higher than 2%, and the carrier satisfies a ratio (S1/S2) of surface area (S1) measured by air-permeation in comparison with surface area (S2), a range represented by;1.2≦(S1/S2)≦2.0wherein the S2 represents surface area (S2) calculated from the following;S2=(6/ρ·D50)×104wherein, the ρ is specific gravity of the carrier.
The use of this kind of carrier is stated to provide the following benefits;                (1) Surface area per unit volume is large, therefore they can give a good enough triboelectric-charge for each toner, and scarcely yield toners which have a low level of electric-charge and reverse polarity-charge too, accordingly scattering of toner particles at the periphery of dot for image-forming and smear (blurring) in background area are few, thus dot reproducibility is excellent;        (2) Due to the nature of large surface area per unit volume and low generation of smear in the background area, low level of average electric-charge in the toner is allowable, notwithstanding, high image density is obtained, thus a carrier of small diameter is able to compensate the shortcomings caused by use of small size of toner, and hence is effective for driving out the advantages of small size of toner;        (3) As a small diameter carrier forms a dense magnetic brush, and the head of the formed magnetic brush has excellent fluidity, the trace drawn by dragging of the head of the magnetic brush on the image is hardly imprinted.        
However, carriers of small diameter in the prior art have an important problem in that that they are apt to deposit themselves on surfaces contacted with the developer, thus providing flaws on the photosensitive member or fixing roller. Thus, they are difficult to utilize, and impractical.